I. Field of the Invention
The present invention relates to an improved tape guidance system.
II. Background Art
Computers which read and write data on magnetic tape require rapid acceleration and deceleration of the tape. The belt-driven tape cartridge disclosed in U.S. Pat. No. 3,692,255 (Von Behren) has been found best suited to meet this requirement, however, it is not without its limitations. The belt-driven tape cartridge, first marketed in 1972, was made to meet the density and speed requirements at that time. Since then, the 1/4" wide (6.35 mm), 1 mil (0.025 mm) tape, driven at 30" (762 mm) per second, has become available in widths from 0.150" (3.81 mm) to 0.250" (6.35 mm), is as thin as 0.6 mil (0.015 mm) and may be.driven at 120 or more inches (30479 mm) per second. Data densities have increased by nearly an order of magnitude. The high density of data on the tape is made possible by multiple independent, parallel tracks spaced across the width of the tape. The higher tape speeds and densities have introduced the possibility of the tape drifting up or down with respect to the tape head, causing head to datatrack misalignment.
Traditionally, transverse confinement is accomplished by the guides which have upper and lower shoulders which bear upon the upper and lower edges, respectively, of the tape to prevent the tape from wandering transversely from its intended path. This type of tape guidance, known as "opposed surface guidance", is adequate when the tape position need only be relatively loosely controlled, but is inadequate in the case where it is necessary to record and read a number of parallel adjacent tracks across the width of the tape.
The need for positive confinement of the tape in a direction transverse to tape motion and a proposed solution are disclosed in U.S. Pat. No. 4,534,523 (Zarr). The Zarr patent describes the use of at least three tape guidance surfaces wherein the shoulders of the tape guidance surfaces are arranged alternately along the tape path so that the shoulders of successive tape guidance surfaces bear against opposite edges of the tape, and where the planes containing the shoulders are spaced at a distance equal to or less than the width of the tape. Such a system, known as "alternate edge guidance", is subject to the flatness characteristics of the baseplate due to the requirement that a contact pattern of top-bottom-top, or bottom-top-bottom, be maintained. It is possible to have a cartridge manufactured where the tolerances are all nominal such that the tape guides can no longer provide alternate edge surface guidance, in which case the tracking accuracy of the guides are impaired.
It would be desirable to provide a tape guidance system which utilizes a tape's natural tendency to deviate from a true straight line which is not dependent on baseplate flatness and which does not introduce the risk of damage to the tape edge by attempting to run the tape through a tape guide which is narrower than the width of the tape. It is to this objective that the present invention is directed.